1. Field of the Invention
This invention relates to a device for driving an objective lens in the direction of the optic axis thereof and a direction perpendicular to the optic axis, and more particularly to an objective lens driving device suitable for use in an optical recording-reproducing apparatus which condenses a light beam on a recording medium and thereby accomplishes recording or reproduction of information.
2. Description of the Prior Art
In recent years, along with the advance of the information society, the demand has been rising for optical recording-reproducing apparatuses for recording information on a recording medium such as an optical disc by the use of a light beam or for reproducing the information accumulated on a recording medium. Usually, in such optical recording-reproducing apparatuses, the focus control in which whether the beam spot condensed by an objective lens is accurately focused to the information track on the recording medium is detected and the objective lens is displaced in the direction of the optic axis (the focus direction) on the basis of the detected error signal and the tracking control in which the amount of deviation of said spot from the information track is detected and the objective lens is displaced in a direction perpendicular to the information track (the tracking direction) on the basis of the detected tracking error signal are effected. In driving such an objective lens, a device is necessary in which when the objective lens is to be displaced in the focus direction or the tracking direction, the optic axis thereof is not inclined or the controls in the respective directions do not interfere with each other and which can accurately move the objective lens independently in the respective directions.
As a conventional objective lens driving device of this type, there has been proposed a device in which an objective lens is mounted at a position eccentric from the center axis of an objective lens holding cylinder and a fixed shaft projecting from a portion of an objective lens driving device housing is passed through the center axis of the objective lens holding cylinder and the objective lens holding cylinder is rotated about said fixed shaft, whereby the objective lens is moved in the tracking direction and by causing the objective lens holding cylinder to slide along said fixed shaft, the objective lens is moved in the focus direction (EP No. 68757).
In such an objective lens driving device, a coil for imparting a drive in the focus direction and a coil for imparting a drive in the tracking direction have been provided around the outer peripheral portion of the objective lens holding cylinder and these coils have been disposed so as to cross a magnetic circuit comprising a yoke and a permanent magnet provided inside and outside the objective lens holding cylinder. Also, the objective lens holding cylinder has been designed to be held at a neutral position in both the focus direction and the tracking direction by a restitution spring formed of a visco-elastic material such as rubber.
However, the conventional objective lens driving device as described above has suffered from a disadvantage that when the objective lens is driven in the focus direction, the holding cylinder slides relative the fixed shaft and therefore a friction force is created to cause a stick slip phenomenon or the sliding resistance is remarkably increased by extraneous conditions such as dewing and the like and in the worst case, the device becomes inoperable. Also, when the entire optical recording-reproducing apparatus is inclined, instability of the focus control operation is caused by the increase in friction force resulting from the difference in posture, and this has sometimes led to the deterioration of the control performance. Further, since the restitution spring is comprised of a single member, it has been difficult to make a design wherein both of the spring constants in the focus direction and the tracking direction are made into optimum values, and there has also been a disadvantage that the displacements in the respective directions influence each other through the restitution spring and adversely affect the control performances in the respective directions.